Supercharged gasoline combustion engines may be operated using variable compression systems. The compression ratio, denoted by ε, may be varied in the range between 8 and 14. It may be desirable to operate the individual cylinders of a gasoline combustion engine using maximum available compression in the lower part throttle range, and using minimum available compression in close to full throttle operating ranges, so as to limit the tendency to knocking that is pronounced there.
A mechanical solution for attaining a variable compression in an Otto engine is discussed in German Patent Application No. 198 44 200. The crankshaft drive suggested there for the variable compression of a reciprocating combustion engine or reciprocating compressor is made up of a connecting rod having an adjustable position of the small end bearing. The length of the connecting rod, i.e., the distance between the axes of the small end bearing and the piston pin bore bearing may be changed by the use of two or more hydraulic chambers. For this purpose, the hydraulic chambers are configured in different but compensating dimensions, whereby bores running inside the crankshaft and the operating piston and used for setting the compression and possible lubrication are provided with lubricating oil, one of the two hydraulic chambers of a pair of chambers being able, in principle, to be replaced by a return spring. Between the hydraulic chambers mentioned, a double-acting operating piston is accommodated, which, in turn, accommodates the small end bearing.
A further device for changing the compression of a reciprocating combustion engine is discussed in German Patent Application No. 197 03 948. This device for changing the compression in a reciprocating combustion engine includes a crankshaft rigidly supported in the engine housing, a connecting rod supported on a crank of the crankshaft, and a piston supported on the connecting rod and able to move up and down inside a cylinder fixed in the engine housing. Also included is an eccentric sleeve which is supported on the crank by its cylindrical inner surface, and is supported on the connecting rod by its outer surface which is eccentric with respect to the inner surface. The effective length of the connecting rod may be changed by the rotation of the sleeve relative to the connecting rod. The eccentric sleeve is configured to include at least two locking recesses, a locking member being mounted on the connecting rod, which, when moving in one direction, engages with one locking recess of the sleeve, and when moving in another direction engages with the other locking recess of the sleeve, the one locked rotary position of the sleeve generally corresponding to the maximum effective length of the connecting rod and the other locked rotary position of the sleeve generally corresponding to the minimum effective length of the connecting rod.
The compression ratio of Otto engines has a decisive influence on the operating behavior of supercharged Otto engines. In the operation of Otto engines having high loads, lowering the compression ratio is the usual manner of controlling the engine's tendency to knock. At too high a rating of the compression ratio, full throttle operation is limited by knocking, and the ignition angle has to be set back. Setting back the ignition angle results in a loss of efficiency, which in turn limits the attainable full throttle mean pressures. The late combustion phase relations connected with setting back the ignition angle result in high exhaust gas temperatures. In turn, reducing the exhaust gas temperature, which is required with regard to component part protection, results in a great mixture enrichment, which, however leads to additional consumption difficulties.